As is well known in the art, no lens stop is provided for a lens utilizing a reflex optical system, and therefore an ND filter for optical intensity adjustment or an ordinary filter is disposed behind the optical system. These filters may be built in a large lens utilizing a reflex optical system. In order to reduce the size and weight of such a lens, the filter is screwed into or inserted into the rear part thereof. However, this method is disadvantageous in that it is troublesome to exchange the filter or the filter may be lost.
In the case of a zoom lens utilizing a reflex optical system, the diameter of the bundle of rays in the rear part thereof is changed by zooming, and therefore a filter having a larger diameter is needed. Heretofore, it has been difficult to build filters small in size and weight in a reflex telephoto lens.